Various techniques have been employed in the prior art to transport a reticle past an optical slit with high accuracy. One such approach moves the reticle arcuately by mounting it on the end of a pivoted arm which is then driven back and forth by a motor.
Another technique involves moving the reticle linearly by means of an air bar. An air bar may be thought of as an elongated bar of square cross section having precision lapped surfaces. Riding along the air bar is a square tube which carries the reticle. Pressurized air between the air bar and the tube provides substantially friction-free movement. However, with both the air bar and the pivoting arm approach, the number of possible adjustments of reticle position were limited.